The present invention relates to the production of printed wiring boards (PWB) using polyetherimide substrates and more particularly to improving the peel strength of the copper foil bonded thereto.
A variety of substrates have been used in the production of copper-clad laminates for printed wiring boards, including woven fabrics, non-woven fabrics, paper-like materials made from glass fibers, organic fibers, wood pulp, and the like with various thermosetting resins including phenolic resins, epoxy resins, polyester resins, and the like. Specific structures include, for example, paper-phenolic resin, glass mat-polyester, glass cloth-epoxy resin, and like systems. The laminates generally have been produced by impregnating or coating a substrate with a varnish which is liquid at room temperature and has been diluted with a solvent, removing the solvent in a dryer, and simultaneously proceeding the reaction to a state suitable for press molding in a later stage. This produces a so-called pre-preg wherein the varnish has proceeded to the B stage. The pre-pregs can be cut to dimension, copper foil placed adjacent one or both surfaces of the pre-preg, and the multi-layer structure sent to pressure molding operations for ultimate cure of the pre-preg and its bonding to the copper foil.
A variety of alternative manufacturing procedures have been proposed in the art. For example, the so-called SMC process utilizes glass chopped strands which are impregnated with an unsaturated polyester resin which has its viscosity chemically increased for handling. A copper foil is affixed to the surface of the impregnated mat and the resulting structure supplied to a heated pressure mold. The pultrusion process for producing a laminate comprises passing a substrate through an unsaturated polyester resin bath for impregnation with the resin, supplying the substrate together with the copper foil into a heating die, and conducting continuous molding. British Pat. No. 1,525,872 proposes the use of a solventless resin bath for impregnation of the reinforcement and conducts molding by calendering or heating roller press operation. Japanese Kokai 13589/79 proposes the formation of a mixture of solventless resin and filler into a sheet-like material using an extruder followed by conducting a reaction to the B stage. Application of the copper foil and molding then proceeds, e.g. using a belting press. Finally, U.S. Pat. No. 4,510,008 proposes to coat a solventless thermosetting resin onto one surface of a fibrous substrate. Copper foil then is placed in contact with the coated side of the substrate and the laminate subjected to pressure molding.
In making copper-clad laminates utilizing polyetherimide substrates, optionally reinforced, U.S. Pat. No. 3,682,960 proposes a solvent solution of curable polyamic acid and an amide-modified polyamic acid which is coated on the metal foil and cured. This same type of coating approach for polyetherimides, polyamides, polyimides, and like substrates is proposed in U.S. Pat. Nos. 4,562,119, 4,374,972, and 4,433,131. Adequate peel strengths of the copper foil are reported in these citations. However, when a pre-formed polyetherimide extrudate is placed in contact with copper foil and the laminate sent to a heated press, copper foil peel strengths of the resulting cured laminate are insufficient for a variety of industrial applications.